An electric linear motion actuator of this type includes a motion convertor mechanism which converts the rotary motion of the rotor shaft of an electric motor to a linear motion of an axially movable driven member.
Known motion convertor mechanisms used in this type of electric linear motion actuators include a ball-screw mechanism and a ball-ramp mechanism. These motion convertor mechanisms can increase power to some extent but cannot increase power to such an extent as required in an electric disk brake system.
Thus in an electric linear motion actuator using one of the above-mentioned motion convertor mechanisms, a reduction mechanism such as a planetary gear mechanism is additionally provided to increase driving force. Such a separate reduction mechanism adds to the complexity and the size of the electric linear motion actuator.
The applicant of the present invention has already proposed in JP 2010-65777A and JP 2010-90959A electric linear motion actuators which are free of this problem, and which can sufficiently increase power without the need for a separate reduction mechanism and thus can be used in an electric disk brake system, of which the linear motion stroke is relatively small.
The electric linear motion actuator disclosed in either of JP 2010-65777A and JP 2010-90959A includes a rotary shaft rotated by an electric motor, an axially movably supported outer ring member, and planetary rollers mounted between the rotary shaft and the outer ring member. A helical rib is formed on the radially inner surface of the outer ring member which is engaged in helical grooves or circumferential grooves formed on the radially outer surfaces of the respective planetary rollers. Thus when the rotary shaft rotates, the planetary rollers revolve around the rotary shaft while rotating about their respective axes due to contact friction between the planetary rollers and the rotary shaft, and simultaneously, the outer ring member is moved in the axial direction.
If the electric linear motion actuator disclosed either JP 2010-65777A or JP 2010-90959A is used in an electric disk brake system, such a disk brake system has the function as the service brake only, i.e. the brake which can only control the braking force according to the force applied to the brake pedal by a driver. In order to keep the vehicle parked in position, it is necessary to keep the electric motor energized while the vehicle is being parked, which is a huge waste of electric energy.
JP 2006-183809A proposes an electric brake system which includes a parking brake locking mechanism which, when actuated, is adapted to stop rotation of the rotor of the electric motor in the brake releasing direction. This almost completely eliminates the disadvantages of the above-mentioned disk brake system.
While the electric brake system disclosed in JP 2006-183809A has the parking brake locking mechanism, since this locking mechanism is mounted around the rotor, the brake system tends to be large in diameter, so that the brake system could interfere with a wheel when mounted on a vehicle.
Also, since the parking brake locking mechanism is mounted around the rotor, it adds to the weight of the electric motor, resulting in a weight imbalance of the entire brake system, which could detrimentally influence how the brake pads are pressed against the brake disk during braking, thus destabilizing the braking operation. This could cause brake squeak.
In the electric brake system disclosed in JP 2006-183809A, various components of the parking brake locking mechanism, such as a pivot arm with a claw, a solenoid for pivoting the pivot arm and a torsion return spring for the pivot arm, are formed into a module so that these components can be easily mounted on the brake system. A rigid protective cover is necessary to protect this module against e.g. flying stones while the vehicle is traveling.